Diamond wire slicing is a technology that is being adopted for photovoltaic (PV) silicon wafer manufacturing. Different from loosen abrasive wire saw technology, diamond wire fixes abrasive grains on a core wire with a resin layer or by electroplating and performs cutting action through the fixed abrasive grains. The slicing process includes moving the diamond wire saw against the work piece, e.g., a silicon ingot, while a cutting fluid or coolant is sprayed onto the wire web from a storage tank. The liquid film formed on the wire web or wires travel with the moving wires to the contact front of the work piece to provide cooling and lubrication. The cutting fluid then falls back to the storage tank together with work piece powders or particles generated from the slicing process. The cutting fluid mixture is cooled and circulated back for continuous use until the cutting fluid becomes exhausted or the content of powders reaches a certain level. The temperature of the cutting fluid or the mixture of cutting fluid and powders is maintained at or slightly below room temperature, e.g., 25° C. At the contact surface of wire with a silicon ingot, the temperature typically ranges from 50° to 80° C. due to the friction between ingot and wire. In addition to the primary functions of cooling and lubrication, the cutting fluid should also provide suspension and carrying (i.e., dispersion) capability of the work piece powders (swarf), and it should generate little, if any, foam.
Water-based cutting fluids are desired for diamond wire wafer slicing because they provide good cooling efficiency and less environmental impact, and they offer the potential for lower cost. However, technical challenges exist that prevent water-based cutting fluids from being practically acceptable. The major challenges include wafer surface cleaning difficulties and hydrogen generation, which are typically associated with the reaction of a freshly generated silicon surface with water. In addition, the lubricity of water-based cutting fluids is inferior to polyalkylene glycol (PAG) based cutting fluids.
Of interest to practitioners of diamond wire cutting technology, particularly to those who use this art to cut silicon ingots, is a water-based cutting fluid that exhibits good lubricity and dispersion capacity yet minimizes hydrogen production and wafer cleaning issues.